"Fetus in fetu" is a rare medical condition in which a malformed or abnormal fetus is found inside the body of its twin. Although there are no direct synonyms for this term, there are related medical conditions that may be used interchangeably. These include teratoma, which is a type of tumor that may contain tissue from different organs, and parasitic twin, which is a condition in which a conjoined twin is partially absorbed by the other twin's body. While these terms may not precisely define "fetus in fetu," they are often used to describe similar medical conditions. It is important to note that such conditions are extremely rare and require prompt medical attention.